Optical cross-connect switch with telecentric lens and multi-surface optical element

ABSTRACT

In the beam path of an Optical Cross Connect between the front face of a fiber block and a moveable mirror array are placed a telecentric lens and multi-surface optical element. The lens is placed adjacent the front face with a front focal plane coinciding with the front face. The substantially parallel beam path axes between the front face and the telecentric lens are converted by the lens into dispersing directions towards the optical element. Discrete optical surfaces of the optical element redirect the dispersing beam paths in a fashion such that the beam paths coincide in the following with corresponding moveable mirrors of a mirror array. Pitches of arrayed fiber ends and of the optical surfaces as well as the moveable mirrors are independently selectable. The telecentric lens simultaneously focuses the signal beams with improved beam separation and reduced signal loss.

CROSS REFERENCE

This application is a divisional of U.S. patent application Ser. No.10/354,887, filed on Jan. 29, 2003, and claims priority therefrom under35 U.S.C.§ 120. The priority application is currently pending. Thisapplication also cross-references and hereby incorporates by referenceU.S. patent application Ser. No. 10/354,901, entitled “ASSEMBLEDMULTI-SURFACE OPTICAL COMPONENT AND METHOD FOR FABRICATING” and filedJan. 29, 2003.

FIELD OF INVENTION

The present invention relates to Optical Cross-connect Switches (OXC).Particularly, the present invention relates to OXC with combined signalbeam focusing.

In the field of optical telecommunication, Optical Cross-connectSwitches (OXC) provide simultaneous switching of up to several thousandsof signal beams. Generally, there is a continuing demand for OXC thatcan be more efficiently fabricated with ever increasing numbers ofsimultaneously switched signal beams.

For best understanding of the improvements provided by the presentinvention, a prior art OXC 100 may be initially described by referringto prior art FIG. 1. The prior art OXC 100 includes a bundle of incomingfibers 101 and outgoing fibers 112 that pass through a laser card 102where a laser light is inserted and aligned with each beam path. Laserlight is propagating in the same direction with the telecommunicationsignals along the input fibers 104. The laser light is propagating inthe opposite direction with the telecommunication signals along theoutput fibers 111.

The fibers 104, 111 terminate with their fiber ends 208 (see FIG. 2) atthe front face 113, 114 of the fiber blocks 105, 110, as is exemplarilydepicted in FIG. 2. The signal beams emit/impinge at the front faces113, 114 substantially collinear with the emitting laser beams. Eachpropagating signal beam is separately reflected together with its laserbeam by a moveable mirror arrayed in the mirror arrays 106, 108. Themovement of the moveable mirrors is coordinated such that a switching ofthe signal beams is accomplished by spatially reorienting the beam pathsbetween the mirror arrays 106, 108.

The laser beams injected by the laser card are deflected by the moveablemirrors in the same fashion as the signal beam. A dichroic flat 109placed in the beam paths between the mirror arrays 106, 108 isconfigured for filtering the laser light by reflecting only the signalbeams. The laser light impinges a PDA detector 103 placed immediatelybehind the dichroic flat 109. The PDA detector 103 recognizes theimpinging coordinates of the lasers, which is part of a feedback looputilized by the processor 107 to monitor and control the moveablemirrors. Each telecommunication beam is reflected along its path betweenthe fiber blocks 105, 110 at the mirror array 106, the dichroic flat 109and the mirror array 108.

Light emits from the fiber ends 208 (see FIG. 2) with a certaindispersion angle making it necessary to focus the signal beams beforedirecting it towards the mirror array 106. Also, the switched signalbeams need to be focused and narrowed before impinging the fiber ends208 at the fiber block 110. In the prior art OXC 100, this isaccomplished by separately focusing each signal beam. As is illustratedin the prior art FIGS. 2, 3, a number of lenses 204 is arrayed andpositioned in alignment with their corresponding fiber ends 208. Thelenses 204 are fabricated with high precision into a lens plate 205 thatis fixed with its frame 206 to the main housing 200 after a precisionalignment procedure.

As can be seen in prior art FIG. 3 there are certain design limitationsassociated with the use of a lens plate 205. Signal beams emit/impingethe fiber ends 208 within the conical beam boundaries 301. To providesufficient spacing for the alignment and positioning, the lens plate 205needs to be spaced apart the end faces 113, 114. In order to capture theentire signal beam, each lens 204 has to have a diameter that at leastequals the lateral extension of a conical boundary 301 where the signalbeam impinges the lens 204. Also, due to precision limitations of theoptical elements in the beam path, the signal beam may have a certainscattering angle with which it propagates towards the opposing fiberblock 110. This may result in an extended width of it, which also needsto be captured by the lenses 204. Hence, the lenses 204 have to besubstantially larger in diameter than the diameter of the fiber ends208.

In addition to the required lens diameter, the lenses 204 need to besufficiently spaced to each other for fabrication purposes. The designrequirements for lens diameter and lens pitch mainly limit the minimalpitch 303, with which the fiber ends 208 are arrayed within the fiberblocks 105, 110. At the time this invention was made, an exemplary pitch303 of a prior art OXC 100 is about 1 mm.

The monolithic fabrication of the small-scale lenses 204 is very costintensive. Each lens 204 has to be fabricated with the same precision.Scaling of an OXC 100 for a larger number of simultaneously switchedtelecommunication signals is limited by the increasing costs associatedwith the fabrication of the lens plate 205.

Due to the small lens sizes, the achievable focusing precision isrelatively low compared to larger size lenses. As a result, the opticalpath between the fiber blocks 105, 110 needs to be kept as short aspossible, which in turn defines the required tilt range of each movablemirror. Unfortunately, the efficiency of the OXC 100 is significantlyinfluenced by the precision and speed with which the two axes tiltmovement of each moveable mirror is accomplished. For that purpose it isdesirable to have the maximum required tilt range of the moveablemirrors at a minimum. At the time this invention was made, an exemplarytilt angle of a moveable mirror is about 8 degrees.

The fabrication of the small scale lenses 204 results also in limitedsurface quality of each lens 204, which in turn induces a certain lossof signal strength. At the time this invention was made, the loss ofsignal strength in an exemplary prior art OXC 100 is about 2 dB. It isdesirable to reduce this loss.

The separate focusing of each signal beam with a monolithic lens arrayrequires also substantially parallel beam propagation between the fiberblocks 105, 110 and their adjacent mirror arrays 106, 108. Thus,dimensional scaling of the mirror array is dependent on scaling of thefiber blocks pitch and lens array.

Finally, the use of a laser card 102 is a cost intensive device forinjecting laser light into the signal beams. Optical elements have to beadditionally provided for separately injecting the laser light into eachsignal line. It is desirable to have an OXC with a configuration inwhich laser light may be injected into the signal beams without need ofa separate laser card.

The prior art OXC design of prior art FIGS. 1, 2, 3 has significantlimitations summarized as follows:

-   -   1) the fabrication of the lens plate 205 is highly cost        intensive since for each fiber end 208 a separate lens 204 needs        to be provided;    -   2) as a consequence of using a lens array, the signal beams        propagate substantially parallel between the fiber blocks 105,        110. This requires simultaneous scaling of fiber blocks 105, 110        and mirror arrays 106, 108;    -   3) miniaturization and scaling of the prior art OXC 100 is        limited by the precision and cost with which the lens array 205        may be fabricated;    -   4) low focusing precision of the lens array 205 requires short        beam paths between the fiber blocks 105, 110 and consequently        increased tilt angles of the moveable mirrors;    -   5) reduced fabrication precision of the lens array results in        loss of signal strength; and    -   6) cost intensive use of a laser card to inject laser light into        the signal beams.

To overcome the limitations described above, an OXC design is needed inwhich:

-   -   1) focusing of the signal beams is accomplished in a simplified        fashion and without use of a micro lens array to reduce        fabrication cost, loss of signal strength;    -   2) scaling and miniaturization of fiber blocks, mirror arrays,        dichroic flat and detector is highly independent of beam paths        and individual elements' design constrains within the OXC; and    -   3) injecting laser into the signal beams without use of a        separate laser card.

The OXC described in the following addresses this needs.

SUMMARY

The OXC of the present invention utilizes telecentric lenses incombination with multi-surface optical elements for a simultaneousfocusing and directing of the signal beams between the fiber blocks andthe moveable mirrors. A telecentric lens is placed adjacent to each ofthe two fiber blocks' front faces such that a front focal plane of thetelecentric lens coincides with the respective fiber block's front face.The telecentric lenses accomplish two tasks simultaneously. Firstly,each signal beam is converted from a dispersing condition towards thetelecentric lenses into a converging condition away from the telecentriclenses. The beams propagate towards the mirror array with convergingbeam widths, which results in improved beam separation and minimal lossof signal strength.

Secondly, each telecentric lens redirects the signal beams' axes from asubstantially parallel direction at the front faces to dispersingdirections such that the distance between adjacent beam path axesincreases with the distance away from the telecentric lenses. The signalpaths coincide centrally with optical surfaces of a multi-surfaceoptical element placed at a certain distance away from the telecentriclens. The pitch of the beam axes at the multi-surface element is amultiple of the pitch with which the signal beams emit/impinge the fiberblock providing for a larger scale fabrication of the optical surfaces.The multi-surface element redirects each beam path separately and in afashion such that each beam path coincides with a single moveable mirrorof a mirror array placed adjacent the optical element. The opticalsurfaces are positioned and oriented in a fashion that corresponds onone hand to the direction of the dispersing path axes and on the otherhand to the mirror arrays' pitch and distance to the optical element.Thus, by introducing a telecentric lens, the pitches of the fiber ends,the optical surfaces and the arrayed moveable mirrors may be separatelyselected.

In the preferred embodiment, the optical surfaces are planar mirrorsthat are easily fabricated. In that context it is referred to thecross-referenced application. Also, the use of a telecentric lensinstead of arrayed micro lenses greatly reduces signal losses andprovides for more flexibility in the configuration of the beam pathsbetween the mirror arrays. As a consequence, the mirror arrays may beplaced in a greater distance to each other, which in turn reduces themaximal required tilt angle of the moveable mirrors. The telecentriclens may be in a commercially available configuration.

The telecentric lens may be configured with a front focal lengthsufficiently long such that a beam splitter may be placed between thefiber blocks and the telecentric lenses. Through the beam splitter, thesignal beams are accessed for laser injection and other monitorfunctions eliminating the need for the laser card.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a schematic view of a prior art Optical Cross Connect[OXC].

FIG. 2 shows a perspective exploded view of a prior art fiber block witha lens plate.

FIG. 3 shows a section view of a prior art fiber block and a lens plate.

FIG. 4 depicts a simplified OXC of the present invention.

FIG. 5 schematically illustrates a portion of the beam paths of an OXCin accordance with the present invention.

FIG. 6 shows a portion of an OXC with a multi-surface optical element inalternate configuration.

FIG. 7 shows a fiber block and a telecentric lens with a beam splitterassembly placed between them.

DETAILED DESCRIPTION

Referring to FIG. 4, an Optical Cross Connect [OXC] 500 of the presentinvention includes a housing 501 is connected to an incoming fiberstring 424 and an outgoing fiber string 425. The optical fibers of theincoming string 424 are inserted in fiber block 420 and terminate at thefiber block's 420 front face 421. The optical fibers of the outgoingstring 425 are inserted in the fiber block 426 and terminate at thefiber block's 426 front face 428. Signal beams emit at the first frontface 421, propagate along the main path 484 through the OXC 500 andimpinge the second front face 428.

The emitting signal beams impinge a first front side 431 of a firsttelecentric lens 430. The telecentric lens 430 is configured in a wellknown fashion to simultaneously transfrom the distinct signal beams'propagation characteristic such that the signal beams emit fromtelecentric lens' 430 first back side 432 with dispersing beam axes 482Aand converging beam widths 486 (see FIG. 5).

Along the main path 484 and following the telecentric lens 430 indirection of signal beam propagation is placed a first multi-surfaceoptical element 440 that has a number of discrete optical surfaces 441.In the preferred embodiment, the discrete optical surfaces 441 areplanar mirrors. Each of the surfaces 441 has a unique position andorientation with respect to preferably one impinging signal beam suchthat all signal beams are independently redirected towards a number ofmoveable mirrors arrayed at the front 451 of a first mirror array 450.Hence, after impinging the surfaces 441, the signal beams propagate awayfrom the first optical element 440 and towards the first mirror array450 along beam axes 482B.

The first optical element 440 provides for an individual redirecting ofeach signal beam. In the preferred embodiment, the beam axes 482A areconverted from a dispersing condition into a converging condition of thebeam axes 482B.

Between first mirror array 450 and second mirror array 460, theswitching of the signal beams takes place by correspondingly actuatingthe mirrors of both mirror arrays 450, 460. The switching of signalbeams takes place by spatially redirecting them while they arepropagating from mirror array 450 to mirror array 460. The spatialredirected beams remain within the boundaries 488 and 489. The signalbeams propagating between mirror array 450 and 460 impinge and arereflected by a dichroic flat 470, which filters control laser beams fromthe signal beams.

The signal beams impinge the moveable mirrors of the mirror array 461and are redirected again towards a second optical element 445 havingdiscrete optical surfaces 443. Between first and second mirror array450, 460, the signal beams propagate within the boundaries 488 alongbeam axes that change as a result of the induced switching operation(s)performed by moveable mirrors. At the second mirror array 460 the signalbeams' axes are again brought into a stable condition with their beamaxes dispersing in constant directions 482C away from the mirror array460. The beam axes 482C are spatially oriented in correspondence to theposition of the second optical surfaces 443 where they are redirectedtowards a second telecentric lens 433. The signal beams propagate fromthe second multi-surface optical element 445 again with converging beamaxes 482D towards the second back side 434 of the second lens 433.

The second telecentric lens 433 induces a simultaneous transformation tothe signal beams in a fashion such that the signal beams emitting on thesecond front side 435 impinge at predetermined locations on the secondfront face 428. The predetermined locations are within the boundaries ofthe fiber ends of the second fiber string 425 and the signal beams areinjected again into the optical fibers of the second fiber string 425.

The upper portion of the OXC 500 including the first fiber block 420,the first telecentric lens 430, the first optical element 440 and thefirst mirror array 450 is preferably symmetrical to the lower portion ofthe OXC 500 including the second fiber block 426, the second telecentriclens 433, the second optical element 445 and the second mirror array460. All signal beams propagate within the OXC 500 within the boundaries481, 486, 487, 488 and 489. The terms upper portion and lower portionpertain to the FIG. 4 and are introduced solely for the purpose of easeof understanding. The telecentric lenses 430, 433 have symmetry axesthat preferably coincide with main path 484.

Now turning to FIG. 5, the geometrical configuration of the upperportion of the OXC 500 is described in detail. Due to the preferablysymmetrical configuration of upper and lower portion, the teachingspresented in the following for the upper portion may be applied to thelower portion with inverted propagation directions of the signal beamsas can be well appreciated by anyone skilled in the art. The fiber endsare parallel arrayed in the fiber block 420 with a first pitch 423.Consequently, the signal beams emit with substantially parallel firstbeam axis 480 and a certain, well-known scattering angle that results inconical beam boundaries 481. Since the telecentric lenses 430, 433provide a simultaneous beam transformation, the signal beams may overlapwhen they impinge/emit the front faces 431, 435. This is particularlyadvantageous because the minimization of the first pitch 423 is nolonger influenced by dimensional limitations of the involved opticalcomponents.

In the present invention the first pitch 423 may be provided with 0.22mm compared to about 1 mm in the prior art. This example demonstratesthe dramatic increase of signal beam density achieved by utilizingtelecentric lenses 430, 433. Reducing the pitch from 1 mm to 0.22 mmincreases the signal beam density more than twenty fold.

Since in the present invention the signal beams may overlap between thefiber block fronts 421, 428 and the lens fronts 431, 435, the distancebetween then may be freely selected. The lenses 433, 430 are accordinglyconfigured such that a front focus of the lenses 433, 430 coincides withthe fiber block fronts 421, 428. The distance 402 may be definedsufficiently large for additional optical component(s) to be placedbetween the fiber block fronts 421, 428 and lens fronts 431, 435. Suchadditional optical component(s) may provide an interaction with thesignal beams similar to that of a well-known laser card and will bedescribed further below together with FIG. 6.

For the purpose of ease of understanding, the main path 484 is shown inFIG. 5 as a straight line with the optical elements 440, 445, the mirrorarrays 450, 460 and the dichroic flat 470 considered as beingtranslucent. Also, the orientation of the optical surfaces 441, 443 isshown in FIG. 5 without regards to their function and solely for ease ofunderstanding of some general dimensional relations of the OXC 500.

The beam transformation performed by the lens 430 includes atransformation from substantial parallel beam axes 480 towards the frontside 431 into dispersing beam axes 482A away from the back side 432. Thebeam transformation performed by the lens 430 also includes atransformation from dispersing beam widths 481 towards the lens front431 into converging beam widths 486 away from the lens back 432.

The beam transformation performed by the lens 433 includes atransformation from dispersing beam axes 482 towards the back side 434into substantial parallel beam axes 480 away from the front side 435.The beam transformation performed by the lens 433 also includes atransformation from converging beam widths 486 towards the lens back 434into dispersing beam widths 481 away from the lens front 435.

The beam widths 486 approach zero towards a reference plane 483, whichis preferably perpendicular to the lenses' 430, 433 symmetry axes. It isdesirable to have each beam's widths 486 equal at the moveable mirrorarrays 450, 460. Hence, the distances 413, 408 between the mirror arrays450, 460 and the dichroic flat 470 are preferably substantially equal.For equal beam widths 486 at the moveable mirrors 450, 460 the referenceplace 483 ideally coincides with the dichroic flat 470.

The most peripheral beam axes 482 are at a certain distance 410 at thereference plane 483. A first scaling rate of the beam axes 481 isdefined as the proportion between the distance 410 and a distance 415between the reference plane 483 and a common point 403 where the beamaxes 482 intersect. The common point 403 is a theoretical point insidethe telecentric lenses 430, 433 and is defined by a well knowncombination of individual lenses lined up inside the telecentric lenses430, 433. The scale of the optical elements 440, 445 is defined incorrespondence to the first scaling rate and a distance 405 of theoptical elements 440, 445 to the common point 403. In the preferredembodiment, the telecentric lenses 430, 433 are preferably substantiallyequal with equally positioned common point 403. As may well beappreciated by anyone skilled in the art, the widths 446 and secondpitch 448 of the optical surfaces 443 are a function of the firstscaling rate, the distance 405 and the number of signal beams alongsecond pitch direction.

Each optical surface 441, 443 has a distinct angular orientation 444,which is defined for its position relative to the main path 484 inaccordance with well-known optical principles for redirecting opticalbeams and in conjunction with optical properties of the optical surface.

A second scaling rate is defined as the proportion between the distanceof the most peripheral impinging locations on the optical elements 440,445 and the distance 409 plus distance 408. A third pitch 452, 463 withwhich the moveable mirrors are arrayed on the mirror arrays 450, 460 isa function of the second scaling rate, the distances 413, 408 and thenumber of signal beams along third pitch direction.

As illustrated in FIG. 5, the optical surfaces 441 are orientated suchthat the beam axes 482B preferably coincide at the mirror array 460.This condition may be applied for the lower portion in the way that thebeam axes 482C preferably coincide at the mirror array 450.Consequently, a maximum tilt angle 407 of the moveable mirrors may be atrigonometric function of the second scaling rate. The maximum tiltangle 407 is thus reduced by reducing the second scaling rate.

Turning now to FIG. 6, an alternate embodiment of the OXC 500 isdescribed. There, the multi-surface optical elements 440, 445 feature areference surface 449 along which the optical surfaces 441, 443 arealigned. The reference surface 449 may have a continuous geometricconfiguration that corresponds to the directional change between beamaxes 482A and 482B in accordance with the physical laws of opticalreflection. For more details refer to the cross-referenced application.

Finally FIG. 7 may be described in more detail. There an embodiment ofthe OXC 500 is depicted in which a beam splitter 701 is placed betweenthe fiber blocks 420, 428 and the lenses 430, 433. The beam splitter 701splits signal beam portions 782 off the signal beams and directs themtowards monitoring device 740, which may be a well-known InGaAs camerafor monitoring signal strength. At the same time, the beam splitter 701injects laser beams 781 coming from a secondary beam splitter 710. Thesecondary beam splitter 710 receives laser pulses 783 from a lasingdevice 720, which may be for example a vertical cavity surface emittinglaser array[VCSEL]. Such VCSEL are commercially available withstandardized pitch 723. Making the first pitch 423 equal to thestandardized pitch 723 is a significant factor for directly insertingthe laser into the signal beams. The amount of additional opticalcomponents may be kept to a minimum.

The secondary beam splitter 710 directs a laser light portion 784towards a laser monitoring device such as a well-known PSD3.

An exemplary OXC 500 in accordance with a preferred embodiment of thepresent invention may have the following characteristics:

path length 484: 1100 mm; throughput loss: 1 db; maximum tilt angle 407:3.5 degrees; first pitch 423: 0.22 mm; number of mirrors/pitch 452 ofmirror arrays 30 × 40/1 mm; 450, 460: mirror tilt accuracy: 0.0015degrees; required angular accuracy of optical surfaces 0.01 degrees;440, 445: mainframe size of housing 501: 10.5 × 6.1 × 3.5 inches.

Accordingly, the scope of the invention described in the specificationabove is set forth by the following claims and their legal equivalent.

Accordingly, the scope of the invention described in the specificationabove is set forth by the following claims and their legal equivalent.

1. An optical system comprising: a. a telecentric lens having aconfiguration for inducing a simultaneous transformation to a number ofdistinctly propagating signal beams from a first beam propagatingcondition into a second propagating condition and vice versa, said firstpropagating condition including substantially parallel first beam axesof said signal beams at a front side of said lens and including beamwidths increasing towards said front side, said second propagatingcondition including second beam axes dispersing in direction away from aback side of said lens and including second beam widths decreasing awayfrom said back side; and b. an optical element having a number ofdistinct optical surfaces and being configured and positioned withrespect to said lens such that at least one of said second beam axescoincides with at least one of said optical surfaces, wherein at leastone of said optical surfaces is oriented such that at least one of saidsecond beam axes is redirected from a dispersing direction into aconverging direction.
 2. The system of claim 1, wherein said opticalsurfaces are planar.
 3. The system of claim 1, wherein said opticalsurfaces are mirrors.